The present invention relates generally to the Stretford process for removing hydrogen sulfide from a gas stream and recovering elemental sulfur as a by-product. More particularly, the present invention concerns an improvement to the gas liquid contacting step by which the Stretford solution is exposed to the gas stream for reaction with the hydrogen sulfide constituent.
Hydrogen sulfide is a noxious gas commonly found in considerable concentrations in sour natural gas and in tail gases from petroleum refineries. The noxiousness of hydrogen sulfide is manifested in a number of ways. For example, an offensive odor is detectable when hydrogen sulfide is present in quantities as low as 0.13 ppm by volume. In addition, a mixture of hydrogen sulfide is present in concentrations as low as 4.4 volume percent. Moreover, hydrogen sulfide is a dangerous mammalian poison.
The noxious character of hydrogen sulfide has led to state and federal laws and regulations that severely restrict the quantities of hydrogen sulfide which may be permissibly exhausted into the atmosphere. In at least partial response to these regulations, numerous processes have been developed to remove hydrogen sulfide from residue gases as well as from otherwise useful products such as natural gas. One of the prominent processes in the petroleum industry for effecting the hydrogen sulfide removal is known as the Stretford process. The Stretford process is generally described in three United States patents issued to T. Nicklin et al, U.S. Pat. No. 2,997,439 issued Aug. 21, 1961; U.S. Pat. No. 3,035,889 issued May 22, 1962; and U.S. Pat. No. 3,097,926 issued July 16, 1963.
According to the Stretford process, gaseous hydrogen sulfide reacts with a solution containing anthraquinone disulphonic acid and an aqueous alkaline solution containing ortho-, meta- and pyro- vanadates of ammonia and alkali metals and a salt of iron, copper, manganese, chromium, nickel, or cobalt. The gaseous hydrogen sulfide, which may be only one constituent of a mixture of gases, is then exposed to the Stretford solution where the reaction occurs. In the reaction, hydrogen sulfide is oxidized by the Stretford solution to elemental sulfur and the solution is reduced. Then air, or an oxygen containing gas, bubbles through the reduced Stretford solution to regenerate that solution by oxidizing it. Subsequently or simultaneously, the sulfur is separated from the Stretford solution and the solution is recirculated in the process. Regeneration of the Stretford solution and separation of elemental sulfur from the Stretford solution typically take place within a flotation cell.
In the typical Stretford process apparatus, the gas/liquid contacting step occurs in a scrubber having a venturi section. As the gas stream containing hydrogen sulfide passes through the venturi, the Stretford solution is sprayed into the gas stream in the form of fine droplets. The chemical reaction discussed above occurs across the surface area of these droplets as they flow downstream along with the gas flow. Generally, gas/liquid contact occurs in a coflowing relationship. Thus, a multiphase flow of gas/liquid and solid particles leaves the venturi discharge.
In conventional Stretford process systems, the multiphase flow from the venturi passes through a conduit that turns the flow to a generally horizontal direction and discharges the flow into the bottom of an absorbing column. The liquid and solid portions of the multiphase flow are generally separated by centrifugal force, as well as by gravity, as the gas stream turns to move vertically upwardly through the absorber column. A second gas/liquid contact step occurs in the absorbing column where additional Stretford solution passes in counterflow relationship with the gas stream and drops to the bottom of the absorbing column.
It is preferred that the primary gas/liquid contact (and sulfur removal) occur upstream of the absorber column. This result is desirable because the small passages of the absorber column become clogged if large quantities of elemental sulfur accumulate therein. Moreover, the large open venturi discharge conduit does not clog when large quantities of the sulfur pass therethrough. Generally speaking, as much as 95-98% of the sulfur should be removed from the gas before it enters the bottom of the absorbing column.
When the Stretford process is scaled up for large volumetric throughflows, the venturi throat diameter may be 24" or greater (i.e. a large venturi). Moreover, it has been observed that inadequate gas/liquid contact occurs in the venturi section. In these large venturis, as little as 65% of the desired sulfur removal occurs.
In general, various kinds of scrubbing devices are known to the art for removing material from a gas stream by use of a liquid. In one such scrubber, a waste gas is contacted with a liquid as the gas passes through a venturi. Downstream of the venturi, the gas and liquid pass through a vertical conduit having twisted helical plates therein. U.S. Pat. No. 3,767,174 issued Oct. 23, 1973 to Heeney. Simple helical vanes at the end of a converging channel have also been used in gas scrubbers. U.S. Pat. No. 818,891 issued Apr. 24, 1906 to Jones et al.
In another scrubber, liquid is sprayed into a gas stream as the gas stream passes through a plurality of parallel venturi channels. After a considerable flow area enlargement, the gas stream and entrained liquid passes through a series of zigzag vanes which separate the liquid from the gas stream. U.S. Pat. No. 4,140,501 issued Feb. 20, 1979 to Ekman. Downwardly sloped deflector plates in a vertical chamber are also known for the purpose of ensuring intimate gas-liquid contact for gas scrubbing. U.S. Pat. No. 3,993,448 issued Nov. 23, 1976 to Lowery, Sr.
Vertically stacked baffle plates have been used in counterflow dust collectors. U.S. Pat. No. 2,259,031 issued Oct. 14, 1941 to Fisher. Vertically stacked arrangements of disc and donut plates are sometimes used to prevent entrained liquid from flowing upwardly with a gas moving through a scrubber, U.S. Pat. No. 3,696,590 issued Oct. 10, 1972 to Richmond, and to promote heat transfer in counterflowing gas and liquid in a scrubber, U.S. Pat. No. 4,149,901 issued Apr. 17, 1979 to Morales. Vertically stacked liquid sprays are also known for cooling a counterflowing gas and removing a crystalline precipitant. U.S. Pat. No. 2,972,393 issued Feb. 21, 1961 to Bush.
Other prior art gas/liquid contact devices are illustrated by the following patents: U.S. Pat. No. 3,782,080 issued Jan. 1, 1974 to Gallagher; U.S. Pat. No. 2,060,166 issued Nov. 10, 1936 to Bowen; U.S. Pat. No. 4,058,378 issued Nov. 15, 1977 to Saxton; U.S. Pat. No. 3,456,928 issued July 22, 1969 to Selway; U.S. Pat. No. 3,894,853 issued July 15, 1975 to Pike; U.S. Pat. No. 3,761,066 issued Sept. 25, 1973 to Wheeler; U.S. Pat. No. 4,145,193 issued Mar. 20, 1979 to Hegemann; and U.S. Pat. No. 3,009,687 issued Nov. 21, 1961 to Hendriks.
Generally, the known contacting devices utilize a column having a large volume so that gas velocities are low while liquid velocities are high. But, the gas velocities at the discharge from a venturi are high by comparison. As a result, most such devices are not suitable for attachment to the discharge of a venturi section. Even where a contacting device is downstream of a venturi (see the Heeney patent), there is no suggestion of using the device in a Stretford process.
It is clear that the need continues to exist for an improvement to the Stretford process which overcomes problems of the type discussed above.